In the past, aluminum alloys, due to their light weight, have been used for engine blocks for internal combustion engines. In order to provide the necessary wear resistance for the cylinder bores, it has been customary to chromium plate the cylinder bores, or alternately, to apply cast iron liners to the bores. It is difficult to uniformly plate the bores and as a result plating is an expensive operation. The use of cast iron liners increases the overall cost of the engine block as well as the weight of the engine.
Hypereutectic aluminum silicon alloys containing 17% to 19% by weight of silicon possess good wear resistant properties achieved by the precipitated silicon crystals, which constitute the primary phase. Because of the wear resistant properties, attempts have been made to utilize hypereutectic aluminum-silicon alloys as casting alloys for engine blocks to eliminate the need of plated or lined cylinder bores.
It has been found that as the silicon content in an aluminum-silicon-copper alloy is increased to the range of 17% to 19%, the castability of the ternary alloy is adversely effected. As an example, a common hypereutectic aluminum-silicon-copper alloy containing 16% to 18% silicon, 0.6% to 1.1% iron, 4.0% to 5.0% copper, 0.1% manganese, and 0.45% to 0.65% magnesium and balance aluminum, has good wear resistance, as well as a desirable low fraction solids at the eutectic temperature, thereby providing good fluidity. However, this alloy has a wide solidification temperature range, in the neighborhood of 250.degree., which severely detracts from its castability. Further, the alloy contains a substantial amount of copper which reduces the corrosion resistance of the alloy in salt water environments and thus prevents its use for marine engines.
Another commonly used hypereutectic aluminum silicon alloy has a nominal composition of 19% silicon, 0.6% copper, 1% magnesium and 0.4% manganese with the balance aluminum. Again, this alloy has good wear resistance due to the precipitated silicon crystals, but has relatively poor corrosion resistance when subjected to salt water environments.